Semiconductor integrated circuits (ICs) contain individual devices, which are typically operatively-coupled together using metal lines. In most applications, the metal lines are formed on a different level than the devices, separated by an intermediate dielectric, such as silicon oxide or borophosphosilicate glass (BPSG). The most commonly used metal lines are aluminum. Interconnects are formed between individual devices and the metal lines. A typical interconnect between metal layers is composed of a via (i.e. opening) formed in an intermediate dielectric. Similarly, an interconnect between a metal layer and silicon is composed of a contact (i.e. opening) formed in an intermediate dielectric over an active device region. The via is filled with a metal, such as aluminum or tungsten. Aluminum has been preferred to date as an interconnect metal. Aluminum exhibits relatively low resistivity as compared to tungsten and, furthermore, is highly compatible with silicon oxide, which is often used as the insulative material surrounding a via. Furthermore, when metal lines are used, which are composed of aluminum, compatibility between the metal lines and the aluminum interconnect materials is optimized.
Interconnects often further contain a diffusion barrier layer sandwiched between the metal and the active device region at the bottom of the via. Such layers prevent intermixing of the metal and material from the active device region, such as silicon, which extends the life of the device. Passive titanium nitride diffusion barrier layers are the most common diffusion barrier layers. Such layers are typically formed over a refractory metal silicide layer. Titanium silicide is the most commonly used refractory metal silicide due to its relatively low resistivity. The use of titanium silicide between titanium nitride and the active device region is preferred due to its intermediate crystallographic characteristics between those of silicon and titanium nitride. The intermediate crystallographic characteristics prevent increased resistivity resulting from a contact solely between silicon and titanium nitride, whose crystallographic characteristics are very different.
Ideally, interconnects exhibit zero impedance to current flow, as exhibited in an ohmic contact (i.e. those which exhibit linear current vs. voltage characteristics), to provide optimum electrical performance. However, interconnects are not ideal and typically exhibit near linear characteristics at best.
One significant concern in depositing metal into a via is obtaining adequate step coverage of the via, particularly obtaining adequate step coverage is difficult when the vias have high aspect ratios (i.e. a large ratio of height to width of the via), as seen more often as IC densities increase. To mitigate this problem, chemical vapor deposition (CVD) is used to deposit the metal instead of physical vapor deposition (PVD). CVD is more apt to adequately fill high-aspect ratio contact holes than PVD. However, to date, CVD aluminum exhibits rough, nonconformal layers on complex topographies, prior to surface modification. This is undesirable because voids often develop within a via, due to the roughness of the CVD aluminum. Such voids severely increase the resistivity of a contact and degrade device performance by not providing uniform connection across an interconnect.
There is a need for an interconnect structure that effectively utilizes aluminum instead of tungsten. There is a further need for a method for forming a smooth, conformal aluminum layer within an interconnect structure. A method for using aluminum in sub-0.25 micron contact holes is needed in order to optimize future device performance.